Bandwidth-efficient modulation methods with a non-constant envelope are used in base stations of the third and higher generations. It is precisely this fluctuation that leads to the need to operate “quasi-linear” high-frequency power amplifiers with a large back-off operating point in order to meet the high linearity requirements that are usually specified by a standard. Utilizing this operating mode, however, leads to poor efficiency, which is reflected strongly in the production and operation costs of the base station.
There are a number of measures that can be used to improve efficiency under a given set of linearity requirements. This can be achieved by special HF power amplifier architectures, using improved transistor technologies, and by special linearization circuits (which can be realized both in the high-frequency range and in the digital baseband). The overall efficiency, which can be achieved by these state-of-the-art methods, however, is still only about 30%. Accordingly, there is a continued need to improve the efficiency of base station power amplifier and modulation systems.